American chemist and physicist Harold C. Urey studied under Niels Bohr at Copenhagen, and is best known for his 1931 discovery of deuterium (heavy hydrogen, the isotope of hydrogen, with one proton and one neutron in its nucleus). He later said he had hoped that this discovery "might have the practical value of, say, neon in neon signs", but its principle use has proven to be in nuclear fusion reactions. Urey won the Nobel Prize for Chemistry in 1934, and also isolated heavy isotopes of carbon, nitrogen, oxygen, and sulfur, conducted respected research in astronomy, geology, and biology.

During World War I he worked as for the Barrett Chemical Company, preparing toluene for the manufacture of trinitrotoluene (TNT). In 1930 he was co-author of Atoms, Molecules, and Quanta, the first widely-used English-language textbook on quantum mechanics and atomic and molecular systems. He was conducting classified research into development of atomic weapons even before World War II, and became a key figure in the Manhattan Project. Working with a team of scientists, he developed the Urey diffusion method to separate uranium-235 from uranium-238.

Within months of the atomic bombing of two cities in Japan, however, Urey authored "I'm A Frightened Man" in the widely-read Collier's magazine, outlining the dangers posed by this new technology. He became a more politically controversial figure in 1952, when he wrote a letter to President Harry S. Truman in support of his colleagues, Morton Sobell and Julius and Ethel Rosenberg, who had been accused of espionage. He later became active with the Union of Concerned Scientists, expressing concern about the proliferation of nuclear weapons and the safety of nuclear power generators.

The Miller-Urey experiment, conducted by Urey's graduate student Stanley Miller in 1953, showed that numerous amino acids necessary for life can be easily produced by heating and agitating ammonia, hydrogen, methane, and water in an airtight container. This "primordial soup" experiment contributed to now-widely accepted theories explaining the origins of the Earth and other planets.

The son of a Christian minister, Urey became an atheist early in his adulthood. He is the namesake of an asteroid, a lunar crater, and the Urey Prize of the American Astronomical Society, awarded annually since 1984 to honor outstanding achievements in planetary science by a young scientist. He was outspoken in his belief that life on other planets is probable, and that humans cannot possibly be the most intelligent species in the universe.